Support proxying repositories in Github and Bitbucket

[fjmilens3]

The proof-of-concept implementation assumes that all dist urls point to a Github repository. Packagist also supports Bitbucket as a first class citizen via webhooks (https://packagist.org/about) so we should likely support that as a minimum before calling this "done."

Supporting Bitbucket would also match existing user expectations from similar formats (e.g. for Bower, which has a similar index mechanism that also points to Git repositories, we consider Github and Bitbucket support to meet our internal "definition of done" for that format).

If you're a Sonatype employee, consult the Bower implementation for ideas on how to do this. If you're not but you want to work on this, feel free to ping us for ideas on where to get started on this issue.

[fjmilens3]

Note that this may work out of the box since we don't have a hardcoded requirement, but we haven't tested it against anything else.

We should verify that it works for both, particularly since we do assume that we're going to be obtaining a zip file from the upstream repo.

[fjmilens3]

This would be worth investigating once the refactoring for non-Packagist repositories is merged.